In-situ experiment on crosshole resistivity tomography was performed in the tailrace tunnel of an electric power plant. The geological condition in the investigated area consists mainly of black schist and partly of quartz and calcite veins. Electric field in rock mass is distorted under the influence of a cavern. In this study, measured data were corrected taking into account the influence. As a result of back analysis, it was found that there were zones of low resistivity below 200m and that there were some regions of relatively high resistivity. To support these features normal log and borehole TV test were conducted. These geological survey agreed with the results of back analysis, indicating that open cracks corresponed to the low resistivity zone and a quartz vein to the high resistivity zone. Accordingly, resistivity tomography is a useful method for estimating the geological features of rock mass.
Nous avons exeerimente in situ la tomographie de resistance en utilisant un forage à l"interieur d"un canal de fuite d"une centrale. Les sols aux alentours de l"emplacement sur laquel l"experimentation a pris place etaient principalement constitues de veines de schiste noir, et partiellement de quartz et de calcite. Etant donne que le courant electrique passe en tant que courant potentiel à l"interieur de la fondation rochese, il est influence par 1"excavation du tunnel. Après avoir corrige les donnees mesurees sur place et analyse par inversion la repartition de la resistance, nous avons determine une zone de faible resistance de moins de 20Ω m à environ 2 m et 7 mde profondeur par rapport aux parois du tunnel. Par ailleurs, nous avons egalement trouve des endroits dans lesquels la resistance etait partiellement elevee. Nous avons ensuite creuse un forage dans la zone d"experimentation afin d"analyser les resultats par diagraphie normale et par sandage televise. Les resultats de l"analyse ont coïncide avec la zone de faible resistance due à la faille et la zone de haute resistance due à la veine de quartz, mis à jour par la diagraphie nomale et par le sandage televise.
Ein-situ Experiment mit vergleichender Widerstandstomographie in Bohrlöchern von Wasserentsorgungstunnein von Kraftwerwerken wurde durchgefuehrt. Die Bodenbeschaffenhei t in der Umgebungdes in-situ Experiments war im wessentlichen von Schiefer bestimmt, wobei teilweise auch Einschließungen von Quartz and Kalzitadern beobachtet wurden. Da der Stromfluß im Felsmassiv zu einem Potentialfluß wird, wird er von Kavetnen beeinflußt. Hier wurden bei der Korrektur in-situ Meßdaten durch Inversion der Widerstandsverteilung um 2m und um7m Tiefe der Tunnelwand herum niederresistente Streifen mit einem Widerstand von weniger als 20Ω m festgestellt. Ferner wurde partielle Widerstandstomographie an hohen Stellen festgestellt. Als nachstes wurden zum Zwecke des Beweises der Analyseergebnisse im Untersuchungsgebiet Bohrlöcher gebohrt, und mit Hilfe von Normaischichtuntersuchung oder Bohrloch TVdie Analyseergenisse blfiesen. Ais Ergelmis wurde mit Normalschichtuntersuchung oder Bohnloch TVklar gezeigt, daß die durch Risse verursachten niederresistenten Bander und die durch Quartzadern verursachten hochresistenten Bander gut uebereinstimmen.
With recent advances in computer technology, geographical tomography(a picture-processing technique for geological structures) has improved the precision of rock mass evaluation. It has been applied as a resource exploration and has also been used for geological investigation for civil work.
Resistivity tomography is a geological tomographical technique that employs an electrical exploration. Its procedure is as follows:
To gather as much data as possible by arranging a large number of electrodes both on the ground surface and in boreholes that surround the region to be investivated; and
To obtain resistivity distribution with high precision and high resolution using back analysis of the data.
The authors have developed computer program of crosshole resistivity tomography, and have been successful in the numerical experiment.
In this study, in-situ experiment was performed to verify this program. Such measurement was performed in an electric power station"s tailrace tunnel, which has seldom been made in a tunnel in the past. Electric field in rock mass is distorted under the influence of a cavern. Due to the substantial effect of a cavern on electric field, adequate correction was made for this effect on measured value.
